The present invention relates to hot mix asphalt facilities, and, more specifically, to a new air handling system for hot mix asphalt batch plants. Prior art hot mix asphalt facilities typically are either the continuous xe2x80x9cdrum mixingxe2x80x9d process or the batch tower (or xe2x80x9cbatch mixingxe2x80x9d) process. In a typical xe2x80x9cdrum mixingxe2x80x9d process, the aggregate enters a drum mixer/dryer at one end of the drum, where it is dried. It progresses along the drum to a point where liquid asphalt is introduced, and then it is mixed with the liquid asphalt on a continuous basis as it progresses toward the other end of the drum. Recycled asphaltic product (RAP) may also be added within this drum mixer/dryer. The finished product is then conveyed via an elevator to a storage silo, where it awaits transfer to trucks, railroad cars, or other means of conveyance to the end user. A single air handling system is used in a xe2x80x9cdrum mixingxe2x80x9d process plant, pulling air through the dryer/mixer.
Prior art hot mix asphalt batch plants typically dry the aggregate in a drying drum. The hot, dry aggregate is then conveyed to a batching tower, where it is held until it is to be weighed out and mixed with liquid asphalt and then conveyed to a silo. These batch plants also use a single air handling system. An exhaust fan draws combustion gases and air from the dryer as well as drawing what is referred to as xe2x80x9cfugitivexe2x80x9d air from the batching tower and the rest of the facility, through one or more dust collectors, and then exhausting the air to the atmosphere. Fugitive dust is the dust that is produced in other parts of the facility outside of the dryer, such as the batching tower, bucket elevator, and mixer. By continually pulling a vacuum on the various parts of the asphalt plant, fugitive dust is prevented from leaking out into the atmosphere and is instead pulled through the single air treatment system.
The dryer includes a burner which requires a proper xe2x80x9cdraftxe2x80x9d or flow of air for its operation. The presence of too much air causes unnecessary cooling of the air through the dryer, stealing drying capacity from the dryer and therefore reducing the efficiency of the facility. The presence of insufficient air causes the burner to xe2x80x9cpuffxe2x80x9d, allowing hot gas to escape and/or leading to a loss of ignition. If the burner puffs out and gas enters the dryer, re-ignition may even cause an explosion in the dryer.
Having a single air handling system for the hot mix asphalt facility appears to make sense from an initial investment point of view. The air from the dryer, as well as the fugitive air, is hot, requiring a dust collector such as a bag house equipped with relatively expensive dust bags capable of handling these high temperatures. A single bag house minimizes the capital investment in this non-revenue producing area of the facility, which appears to be a cost-effective and sensible approach. However, the present invention takes advantage of information that is overlooked by experts in the industry; namely, by recognizing that the use of a single air handling system prevents the operational optimization of either the drum dryer or the fugitive air handling system, thereby making the plant much less efficient and less productive than it could be.
The present invention goes counter to the accepted wisdom of the efficiency and practicality of using a single air handling system in a hot mix asphalt batch facility. Instead, the present invention provides two separate air handling systemsxe2x80x94one for the dryer and one for the fugitive airxe2x80x94so that each of these air handling systems may be operationally optimized, thus improving the operating efficiency of the entire hot mix asphalt facility.